(a) Field of the Invention
The present invention relates to a field emission cold cathode and a method for manufacturing the same. More particularly, it relates to a field emission cold cathode having a reduced parasitic capacitance between a cold cathode and a gate electrode, and a method for fabricating such a field emission cold cathode.
(b) Description of the Related Art:
C. A. Spindt et al. have made an experimental field emission cold cathode having a thin-film structure formed on a silicon substrate by micro-machining using an LSI fabricating techniques (Journal of Applied Physics, Vol. 47, No. 12, 1978). FIG. 1 is a sectional view showing the structure of the cold cathode as described by Spindt et al. An insulating layer 2 having a thickness of 1 .mu.m and a gate electrode 3 made of molybdenum are formed consecutively on a silicon substrate 1. Cavities each having a diameter of about 1.5 .mu.m are formed in the insulating layer 2 for receiving emitters 4 and openings corresponding to the cavities are formed in the gate electrode 3. Each of the conical emitter 4 made of molybdenum is formed in a height of about 1 .mu.m in corresponding one of the cavities so as to make ohmic contact with the silicon substrate 1.
When a voltage ranging from a few tens of volts to 200 volts is applied between the silicon substrate 1 and the gate electrode 3 such that the gate electrode 3 has a positive potential, an electric field of 10.sup.7 V/cm or more is generated at the tip of each emitter 4, the electric field permitting the tip of each emitter 4 to emit electrons to an anode disposed opposite the cold cathode. The conical shape of the emitter causes a difficulty in obtaining a large height thereof, hence, a field emission cold cathode has a small distance between the substrate 1 and the gate electrode 3.
The electron emission of 100 .mu.A or more per an emitter was observed by a recent experiment, and various applications of the field emission cold cathode have been proposed. Examples of the proposed applications include a switching device using a micro-triode in which a field emission cold cathode having a thin film structure is employed as an electron source, and a display panel on which a fluorescent material is made luminous by electrons emitted by a flat emission source including a plurality of emitters of a cold cathode arranged in a matrix.
In a field emission cold cathode as described above, however, a parasitic capacitance is generated between the cold cathode including the substrate 1 and the gate electrodes 3. The parasitic capacitance is increased because of the small distance between the substrate 1 and the gate electrode 3 as well as the enlarged area of the gate electrode including the bonding area and wiring area therefor. When a high frequency voltage is applied between the substrate 1 and the gate electrode 3, the impedance between the substrate 1 and the gate electrode 3 is lowered because of the presence of the large parasitic capacitance, thereby degrading the function of the field emission cold cathode in a high frequency range when applied in a switching device or display panel.